In an attempt to get off the topic of whether Pikachu should have evolved or not . . .
I heard that Team Rocket tried to steal Ash's Thunderstone so that he couldn't use it to evolve Pikachu, and they had it at the end of the episode . . .
That would mean that one of Team Rocket's thefts actually SUCCEEDED! I believe this is the actual first time they had something they tried to steal at the end of an episode (that wasn't a two-parter). This is AMAZING. The writers finally broke the pattern Team Rocket followed for 500 episodes! Will wonders never cease . . . First Jessie winning a contest, and now this!
I really hope they did get the Thunderstone in a theft . . . I've been hoping the writers would do something like this one of these days, but despaired that they ever would. Anything can happen in Diamond and Pearl indeed.